Modeling Scattering-Assisted Transport in Nanodevices
Tuesday, September 20, 2011 – 3:00pm
George Washington University
Inorganic semiconductors have traditionally dominated as the material players in the electronics industry. While their organic counterparts have been studied extensively as alternate channel materials, the development of a stand-alone molecular electronics technology has been stymied by the inordinate difficulty of contacting small molecules reproducibly, their insufficient mobilities, large RC constants and poor gateability. Perhaps a more promising approach is to envisage hybrid organo-semiconductor devices, combining the established infrastructure of the semiconductor integrated industry with the ‘bottom-up’ self-assembly and chemical tunability of molecular monolayers. The uniqueness of this approach lies in the fact that the molecules attach to the surface of a transistor channel and act as scatterers, and not as conductors. The challenge we face is thus to couple the current in the macroscale transistor with strongly correlated molecular scatterers. There are different ways in which a molecule on the surface of a channel ‘talks’ to the channel. The simplest role of molecules grafted on a semiconductor surface is a combination of dipolar electrostatics, and charge-transfer ‘doping’ leading to band-realignment of the underlying silicon FET. Another way that a nanoscale quantum dot can interact with a microscale channel is through its covalent bonding or tunnel coupling, wherein the electron can actually access an alternate coherent pathway through the dot and interfere with the channel current. Finally we study how the overlap of molecular and silicon wavefunctions serves to passivate existing surface states as well as to create new localized molecular trap levels inside the silicon band-gap. At resonance driven by a gate, the traps are stochastically filled and emptied by the channel electrons, blocking and unblocking the channel. We study the resulting switching kinetic behavior called random telegraph signals (RTS), and develop theoretical models to extract trap parameters like depth profiles and energetic locations. Our models couple a density functional treatment of the molecular levels embedded in a semiempirical treatment of the silicon surface with a nonequilibrium Green’s function (NEGF) treatment of quantum transport. We develop atomistic models of contact interfaces and trap creation due to bonding chemistry of candidate adsorbate groups. Correlating the RTS signatures with the unique molecular trap ‘bar-codes’ allows us to characterize and detect molecular species. The effect is enhanced in modern nanodevices as they can be fabricated practically defect free with near ballistic levels of operation. The reduced dimension of the channels also means that surface effects are pronounced as the surface-to-volume ratio increases.